This invention relates to switching regulators suitable for use in television applications.
Self-oscillating switching regulators include a power switching device coupled to a source of unregulated direct voltage. The switching device is gated on and off by a control circuit to produce a pulsating direct voltage. A series inductor and shunt capacitor are coupled to the pulsating direct voltage of averaging the pulsations to produce a direct voltage for application to the load to be energized. The control circuit conventionally compares a sample of the direct load voltage with a reference voltage to produce a switch control signal. When the load voltage decreases, the switch is turned on in order to increase the load voltage, and when the load voltage is too high the switch is turned off so that the load voltage may decrease. Stability of such regulators may be assured by a hysteresis function of the control circuit. In such switching regulators, the output voltage level is proportional to the duty cycle of the switch. Thus, the duty cycle tends to vary with changes in the unregulated direct voltage to maintain a constant regulated voltage. With increasing load current, the filter capacitor tends to be discharged more quickly, and the repetition rate of the regulator tends to rise. A particular disadvantage of such prior art regulators is that the ripple of the output voltage is used to drive the comparator, and therefore the output ripple does not decrease at small output currents.
When used for television applications, it is desirable to synchronize the switching of a switching type regulator with the horizontal deflection so as to eliminate apparent motion on the television screen of transients caused by switching of the regulator. Consequently, the repetition rate of the regulator is held constant, and only duty cycle variations are available to control the voltage to be regulated. In normal operation, a television receiver can be expected to have considerable variations in its current demand, resulting primarily from changes in average kinescope beam current. These changes in load current load the regulator and because the synchronized repetition rate is fixed result in changes in the waveshape of the voltage across the filter capacitor. These changes tend to unsynchronize the regulator by increasing the natural repetition rate or frequency to a value above the synchronizing frequency.